1. Field of the Invention
The present invention relates to a rubber vulcanization agent and a rubber composition containing the same. More specifically, it relates to a rubber vulcanization agent capable of suppressing scorching at the time of processing, of improving the heat stability of the vulcanized rubber, and of facilitating production and a rubber composition containing the same.
2. Description of the Related Art
When the generally widely used sulfur is used as a vulcanization agent for a vulcanizable rubber, a polysulfide bond is mainly formed as the cross-linking structure between carbon chains of the rubber. A bond between sulfurs in polysulfide bonds is susceptible to heat and is easily cut, and therefore, a vulcanized rubber containing a large number of polysulfide cross-linking structures inevitably becomes inferior in the heat resistance. Therefore, as the cross-linking form of the vulcanized rubber, vulcanization agents and vulcanization accelerators for mainly forming monosulfide bonds and/or disulfide bonds have been actively studied. However, most of the vulcanization agents which have been reported to achieve the above object in the past have generated carcinogenic nitroso amines during vulcanization and, therefore, have been problematic in the safety. Further, while the resistance to heat aging of the vulcanized rubber has been improved to a certain extent by the use of the vulcanization accelerator, there has been the problems that the resistance to fatigue aging and resistance to cracking by bending is decreased.
As a rubber vulcanization agent for improving the heat stability of the vulcanized rubber without forming nitroso amines during the vulcanization, a polysulfide polymer containing a large amount of sulfur in the molecular chain has been reported (for example, see (1) Gomu Kogyo Binran (Rubber Industry Handbook, New Edition, p. 167, ed. by Society of Rubber Industry Japan Rubber (1973) and (2) Gosei Gomu Handbukku (Synthetic Rubber Handbook), p. 237, ed. by Shu Kanbara, Kyoichi Kawasaki, Magoichi Kitajima, and Masayuki Furuya (1960)). Further, since a polysulfide polymer is liquid, there is no worry of bloom after vulcanization unlike with vulcanization by sulfur, when used as a vulcanization agent (Japanese Unexamined Patent Publication (Kokai) No. 57-170939). However, a conventional polysulfide polymer is volatile and has a strong odor, and therefore, a vulcanized rubber using this as a vulcanization agent gives a strong odor and is not suitable for practical use (U.S. Pat. No. 2,235,621).
On the other hand, even among polysulfide polymers, since what has been developed as sealing materials have been nonvolatile substances with little odor, the use thereof as a vulcanization agent mixing well with rubber substances is possible. However, when the above liquid polysulfide polymer per se is used as a vulcanization agent, since the end group structure of the polymer is a thiol group, so-called xe2x80x9cscorchingxe2x80x9d where the vulcanization proceeds early at the time of processing easily occurs. Further, since the average number of sulfur bonds in the repeating units is not more than 2, there have been the problems such that a long vulcanization time is required, for example. Thus, to prevent scorching during processing or shorten the vulcanization time, the technique has been adopted of capping the end thiol group in the polysulfide polymer, then increasing the number of sulfur bonds in the repeating units (Japanese Unexamined Patent Publication (Kokai) No. 10-120788). If a reaction with sulfur is caused to increase the number of sulfur bonds in the main chain, without capping the end thiol group in the thiol-terminated polysulfide polymer, the polymer is an unstable substance which generates hydrogen sulfide in the air and is changed to a highly sticky substance. Therefore, the increase in the number of sulfur bonds in a polysulfide polymer main chain should be carried out after capping the end thiol group. That is, the important process in producing a rubber vulcanization polysulfide polymer is the process for producing a polysulfide polymer by capping the end thiol group of a thiol group terminated polysulfide polymer.
The conventional methods of capping a thiol group of a thiol group terminated polysulfide polymer include the method of reacting a halogenated alkyl compound with a thiol group terminated polysulfide polymer and trapping the halogenated hydrogen produced by an amine compound and the method of capping the end with a halogenated alkyl simultaneously with forming the thiol group terminated polysulfide polymer. In both techniques, amine salts or sodium salts are formed as by-products and the complicated operations of filtering out these salt compounds and washing becomes necessary. Therefore, a polysulfide polymer having a capped end thiol group is capable of producing more simply had been sought.
Further, Japanese Unexamined Patent Publication (Kokai) No. 11-322931 discloses polysulfides having various substituents at the end group thereof. Among those, polysulfides having a benzothiazole group, a thiocarbamyl group, etc. which have vulcanization accelerating effects. In fact, these polar groups can accelerate the vulcanization speed, but the scorch time is shortened and therefore causes big problems in the viewpoint of a practical use thereof.
As mentioned above, the prior polysulfide polymers are produced so as to terminate the end thiol group which causes the scorch and also so as to shorten the vulcanization time. However, when the prior polysulfide polymers having the polar groups cannot provide practically sufficient unvulcanized rubber characteristics and also the properties of the vulcanized rubber are not sufficient.
Accordingly, the objects of the present invention are to maintain the sufficient unvulcanized characteristics (i.e., scorch and vulcanization time) and to easily provide a rubber vulcanization agent capable of improving the properties and the vulcanized rubber and the heat stability thereof.
In accordance with the present invention, there is provided a rubber vulcanization agent comprising a polysulfide polymer having the formula (I):
Rxe2x80x94S"Parenopenst"Yxe2x80x94Sx"Parenclosest"nYxe2x80x94Sxe2x80x94Rxe2x80x83xe2x80x83(I)
wherein Y indicates an alkylene group which may include a hetero atom, n is an integer of 1 to 100, x is more than 2 but not more than 6, and R is a residue obtained by reacting a polysulfide polymer having the formula (II):
HS"Parenopenst"Yxe2x80x94Sx"Parenclosest"nYxe2x80x94SHxe2x80x83xe2x80x83(II)
wherein Y, n and x are as defined above
with (i) an unsaturated alicyclic compound or (ii) a compound having the structure (III) or (IV) xe2x80x83R2OOCxe2x80x94CHxe2x95x90CHxe2x80x94COOR2xe2x80x83xe2x80x83(IV)
wherein R1 indicates a hydrogen atom or a methyl group and R2 indicates a C1 to C18 hydrocarbon group having no polar groups, so as to cap the end thiol group.
In accordance with the present invention, there is also provided a rubber vulcanization agent having the average number of sulfur bonds of 3 to 5.
In accordance with the present invention, there is further provided a rubber vulcanization agent wherein R2 in the formula (III) or (IV) indicates a C3 to C8 alkyl group.
In accordance with the present invention, there is further provided a rubber composition comprising 0.1 to 30 parts by weight of a rubber vulcanization agent, based upon 100 parts by weight of a diene rubber.
In accordance with the present invention, there is still further provided a rubber composition further comprising a sulfur at a weight ratio of the rubber vulcanization agent/the sulfur of at least 0.5, provided that the total amount of the rubber vulcanization and the sulfur is 0.5 to 20 parts by weight based upon 100 parts by weight of a diene rubber.
In this specification and in the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:
The singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise. xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstances may or may not occur, and that the description included instances where said event or circumstance occurs and instances where it does not. Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
The present inventors engaged in intensive studies based on the above prior art and, as a result, found that, when an addition reaction between a polysulfide polymer having a thiol group at its end and (i) an unsaturated alicyclic compound, or (ii) a compound having the above structure (III) or (IV) is carried out so as to obtain a polysulfide polymer having a capped end thiol group, without forming byproducts, then adding sulfur in the main chain of the polymer, it is possible to produce a polysulfide polymer having the formula (I), as a rubber vulcanization agent by a simpler technique, whereby the present invention was completed. Further, we found that by blending 0.1 to 30 parts by weight of the rubber vulcanization agent, or a weight ratio of the rubber vulcanization agent/the sulfur of at least 0.5 and the total amount of the rubber vulcanization agent and the sulfur of 0.5-20 parts by weight, into 100 parts by weight of a diene rubber, the resistance to heat aging of the vulcanized rubber composition is extremely improved.
Further, the end functional groups the rubber vulcanization agent of the present invention can be freely selected from numerous types thereof. By suitably selecting the vulcanization agent, depending upon the type of the rubber, it is possible to improve the solubility or dispersability thereof in the rubber.
The polysulfide polymer having a thiol group at the end used as a material of the rubber vulcanization agent of the present invention can be exemplified by the formula (II):
HS"Parenopenst"Yxe2x80x94Sx"Parenclosest"nYxe2x80x94SHxe2x80x83xe2x80x83(II)
wherein Y preferably indicates a C1 to C24 linear or branched alkylene group, which may contain a hetero atom such as oxygen, sulfur, or nitrogen. For example, as Y, methylene, ethylene, propylene, butylene, pentylene, hexylene, octylene, nonylene, decylene, undecylene, dodecylene, 1-methylethylene, 1-methylpropylene, 2-methylpropylene, 1,1-dimethylethylene, oxydiethylene, methylene-bis(oxyethylene), ethylene-bis(oxyethylene), thiodiethylene, N-methyl-N,N-diethylene, etc. In addition, the trivalent hydrocarbon groups such as 1,2,3-propantriyl group may be included, so far as the viscosity is not remarkably increased.
As specific examples of the polysulfide polymer having a thiol group at the end thereof, when Y is methylene-bis(oxyethylene), for example, the liquid polysulfide polymers made by Toray-Thiokol such as Thiokol LP-3 (average molecular weight of 1000) and Thiokol LP-55 (average molecular weight of 4000) may be mentioned.
A first polysulfide polymer having a capped end thiol group of the present invention can be produced by carrying out an addition reaction between a polysulfide polymer having a thiol group at the end thereof shown in the above formula (II) and an unsaturated alicyclic compound with or without the use of a solvent.
The unsaturated alicyclic compound used in the reaction is not particularly limited so long as it is an unsaturated alicyclic compound containing at least one double bond in the ring in the molecule, but cyclopentene, cyclohexene, cyclohexadiene, cycloheptene, cycloheptadiene, cycloheptatriene, cyclooctene, cyclooctadiene, cyclooctatriene, cyclooctatetraene, norbornene (bicyclo[2,2,1]hept-2-ene), norbornadiene (bicyclo[2,2,1]hept-2,5-diene), and their alkyl or alkenyl, or alkylidene substituents; polybicycloheptadienes such as dicyclopentadiene, tricyclopentadiene, and tetracyclopentadiene and their alkyl or alkenyl or alkylidene substituents, etc. may be exemplified. Among these, 5-ethylidene-2-norbornene(5-ethylidenebicyclo[2,2,1]hept-2-ene), 5-vinyl-2-norbornene(5-ethylidenebicyclo[2,2,1]hept-2-ene), 4-vinyl-1-cyclohexene, tetrahydroindene, and dicyclopentadiene are preferable, since they have in their molecules both a double bond selectively reacting with the end thiol group of the polysulfide and a double bond capable of forming a cross-linking structure with the rubber molecules at the time of vulcanization. In particular, dicyclopentadiene, norbornene, etc. is preferable in that it is used generally and broadly and can be inexpensively obtained.
The reaction between the thiol group terminated polysulfide polymer and unsaturated alicyclic compound may normally be performed by a suitable amount of reaction of the Cxe2x80x94C double bonds in the unsaturated alicyclic compound with the thiol groups of the thiol group terminated polysulfide polymer, but is not limited to this stoichiometric equivalent. The ratio of the thiol group terminated polysulfide polymer and unsaturated alicyclic compound may be suitably determined. (e.g., 1:1.05-1.3)
The solvent usable in the reaction is not particularly limited so long as it uniformly dissolves the thiol group terminated polysulfide polymer and unsaturated alicyclic compound. Esters such as ethyl acetate, propyl acetate, and butyl acetate; ketones such as acetone, methylethylketone, and methylisobutylketone; ethers such as tetrahydrofuran, diethylene glycol dimethyl ether; aromatic hydrocarbons such as toluene and xylene; aliphatic hydrocarbons such as pentane, hexane, and cyclohexane; amide-based solvents such as dimethylacetamide and dimethylformamide; pyrrolidone-based solvents such as N-methylpyrrolidone and pyrrolidone; etc. may be mentioned. Preferably, dimethylacetamide, dimethylformamide, N-methylpyrrolidone, and mixed solvents of the same may be exemplified. The amount of the solvent used is 1 to 20 parts by weight, preferably 1 to 10 parts by weight, based upon 100 parts by weight of the thiol group terminated polysulfide polymer.
The reaction can be performed at, for example, 10 to 150xc2x0 C., preferably 30 to 130xc2x0 C., for, for example, 1 to 36 hours, preferably 3 to 20 hours. If the temperature is less than 10xc2x0 C., the reaction tends to take a long time, while even if the reaction is performed at a temperature higher than 150xc2x0 C., a commensurate effect of improvement of the reaction is not necessarily obtained and therefore, this is not wise economically.
A second polysulfide polymer having a capped end thiol group of the present invention can be produced by carrying out an addition reaction between a polysulfide polymer having a thiol group at the end as shown in the above formula (II) and an unsaturated hydrocarbon compound having the above formula (III) or (IV) in a solvent or without the use of a solvent and in the presence of a catalyst.
As the unsaturated hydrocarbon compound having an electron attracting group used in the reaction, (meth)acrylic acid; (meth)acrylic acid esters such as methyl(meth)acrylate, ethyl(meth)acrylate, ethyl(meth)acrylate, butyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, octadecyl(meth)acrylate, and cyclohexyl(meth)acrylate, benzyl(meth)acrylate; diesters between dicarboxylic acids such as maleic acid, fumaric acid, and C1 to C18 linear or branched alcohols may be exemplified.
The reaction between the polysulfide polymer having the thiol group at the end thereof and unsaturated alicyclic compound may normally be carried out by a suitable amount of reaction of the Cxe2x80x94C double bonds of the unsaturated hydrocarbon compound with the thiol groups of the thiol group terminated polysulfide polymer, but is not limited to this stoichiometric equivalent ratio. The ratio of the thiol group terminated polysulfide polymer and unsaturated hydrocarbon compound may be suitably determined (e.g., 1:1.05-1.3).
The solvent usable in the reaction is not particularly limited so long as it uniformly dissolves the thiol group terminated polysulfide polymer and unsaturated hydrocarbon compound. A solvent used, when reacting the above unsaturated alicyclic compound, can be mentioned. Further, the reaction may be performed in the absence of a solvent as well. The amount of the solvent used is 1 to 20 parts by weight, preferably 1 to 10 parts by weight, based upon 100 parts by weight of the thiol group terminated polysulfide polymer.
As the catalyst used, a tertiary amine compound, an alkali metal alkoxide such as sodium methoxide and sodium ethoxide, a quaternary ammonium chlorinated compound such as tetramethylammonium hydroxide and benzyl trimethyl ammonium hydroxide, etc. may be exemplified.
As the tertiary amine compound, monoamines, diamines, triamines, polyamines, cyclic amines, alcohol amines, ether amines, etc. may be mentioned. For example, trialkylamines such as triethylamine, tripropylamine, and tributylamine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylpropane-1,3-diamine, tetramethyl guanidine, N,Nxe2x80x2-dimethylpiperadine, 1,8-diazabicyclo[5,4,0]-7-undecene (DBU), 1,4-diazadicyclo[2,2,2]octane (DABCO), bis(2-dimethylaminoethyl)ether, pyridine, etc. may be exemplified. These tertiary amine compounds may be used in any combinations thereof.
The amount of the catalyst used is, for example, 0.01 to 5 parts by weight, more preferably 0.05 to 1 part by weight, based upon 100 parts by weight of an end thiol group polysulfide polymer.
The reaction can be carried out at, for example, 10 to 130xc2x0 C., preferably 30 to 110xc2x0 C., for, for example, 1 to 36 hours, preferably 3 to 20 hours. If the temperature is less than 10xc2x0 C., the reaction tends to take a long time, while even if the reaction is carried out at a temperature higher than 130xc2x0 C., a commensurate effect of improvement of the reaction is not necessarily obtained so this is not wise economically.
By reacting sulfur with the polysulfide polymer with a capped end thiol group obtained above in, for example, the presence of an alkali catalyst, it is possible to obtain a rubber vulcanization agent comprised of a polysulfide polymer having an average number of sulfur bonds in the repeating units of 2 to 6.
The method of production of the polysulfide polymer may be a method similar to the method described in, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-120788. The sulfur should be added to give an average number of sulfur bonds in the repeating units of the polysulfide polymer obtained of not more than 6. Addition to give an average number of sulfur bonds of not more 5 is further preferred. If the average number of sulfur bonds is more than 6, the heat stability of the vulcanized rubber falls and scorching easily occurs at the time of vulcanization, and therefore there is a detrimental effect on the scorch time. Therefore, this is not preferred.
The alkali catalyst used may be those illustrated in Japanese Unexamined Patent Publication (Kokai) No. 10-120788, but by using a catalyst such as the tertiary amine compound added when capping the end thiol group of the polysulfide polymer as is, it is possible to easily add sulfur to the polysulfide polymer by just adding only sulfur after the step of capping the end thiol group. The amount of the catalyst used is preferably 0.001 to 1 part by weight, more preferably 0.01 to 0.5 part by weight, based upon 100 parts by weight of polysulfide polymer having a capped end thiol group.
The rubber composition of the present invention contains the rubber vulcanization agent obtained above in an amount of 0.1 to 30 parts by weight, preferably 0.5 to 10 parts by weight, more preferably 1 to 6 parts by weight, based upon 100 parts by weight of the rubber. If less than 0.1 part by weight, a sufficient vulcanizing effect cannot be obtained, while if more than 30 parts by weight, further improvement cannot be obtained and, further, the scorching resistance and other aspects of workability are decreased.
As the diene rubber usable in the present invention, natural rubber and a diene synthetic rubber may be used. These may be used alone or may be used in any blend thereof. As the diene synthetic rubber, for example, polyisoprene rubber, polybutadiene rubber, styrene butadiene rubber, butyl rubber, chlorobutyl rubber, chloroprene rubber, etc. may be exemplified.
Further, it is possible to use sulfur together with the rubber vulcanization agent according to the present invention. The amounts thereof are such that a ratio of the above vulcanization agent/the sulfur is 0.5 or more and the total amount of the vulcanization agent and the sulfur is 0.5-20 parts by weight, based upon 100 parts by weight of rubber.
The rubber composition of the present invention may have suitably blended therein a vulcanization accelerator, filler (e.g., carbon black, silica), softener, plasticizer, antiaging agent, etc. normally blended into rubber.
As the rubber vulcanization accelerator, a thiazole based vulcanization accelerator such as dibenzothiazyl disulfide, N-t-butyl-2-benzothiazolyl sulfenamide, N-cyclohexyl-2-benzothiazolyl sulfenamide; a thiuram based vulcanization accelerator such as tetramethylthiuram disulfide; dithiocarbamates, guanidines, thioureas, xanthogenates, etc. may be exemplified. The vulcanization accelerator is preferably used in an amount of 0.1 to 10 parts by weight, more preferably 0.1 to 5 parts by weight, based upon 100 parts by weight of the rubber.
The rubber composition of the present invention may be produced by mixing the above compounding agents by a known mixer or rubber kneading machine, for example, a roll, internal mixer, Banbury mixer, etc. Since the present rubber vulcanization agent is generally relatively high viscosity liquid, it is preferable to add a rubber composition after premixing with carbon black or silica at a weight ratio of, for example, 1:1, although the present rubber vulcanization agent can be added directly to a rubber composition by, for example, a roll.